Normally in these situations the window is replaced when it becomes too dirty, or some mixture of window replacement, or rotation and gas curtains are used.
In U.S. Pat. No. 7,501,145 the laser power coming through a window in a pulsed laser deposition (PLD) process is monitored inside the chamber in order to adjust the incoming laser energy impinging on the window, which is becoming progressively more opaque, to deliver the same energy to the target material to be sputtered. A disadvantage is that eventually losses through the window are too high to be compensated for. This happens more quickly in systems with high deposition rates.
In WO/2005/0075700 and U.S. Pat. No. 4,784,491 a gas curtain is used to slow and deflect “errant material” from a PLD process away from the laser input window. However, in the case of gas curtains there is a requirement to introduce a gas in to the system in order to produce the gas curtain. This limits the vacuum operating pressure of the system and complicates the system design to make sure that the gas input is sufficient to block the debris effectively, but not too much to alter the process parameters. Also, the gas curtain will still allow some window coating given the mean free paths at vacuum pressures typical in pulsed laser deposition (PLD) and extreme ultra violet (EUV) and other plasma process systems.
In another approach a rotating glass window is used until it's transparency drops below a certain level and a new piece of window is rotated in front of the laser—see, for example, www.pvdproducts.com/products/intelligent_windows.aspx. However, rotating a piece of glass is limited by the increase in surface area provided by whatever the diameter of the glass is. It takes the thin layer of debris and spreads it out over a bigger area.
U.S. Pat. No. 5,490,912 discloses the use of a roll of transparent tape which is rolled between the laser plasma plume and the laser input window in a PLD system. Transparent tape on a roll has the advantage of having a much larger surface area than a rotating glass window, but it has the disadvantage that the plastic films are typically damaged or distorted by the laser, especially as debris begins to deposit and laser coupling becomes more efficient. Thus the tape must be moved quickly, and the roll must be rather long, with all the mechanical issues that come with it. Also, for use in a high temperature process environment, plastic tapes are unsuitable as they will distort.
The use of an oil to protect and coat the surface of an optical component has been suggested.
Chraplyvy—Applied Optics, Vol. 16, No. 9, September 1977—“Liquid Surface Coating For Optical Components Used In High Power Laser Applications” suggests using viscous paraffin oil and discusses protecting vertical surfaces but to avoid oil running off increases its viscosity until it becomes gel-like. It is not replenished from a bath.
Kessler—International Symposium on Plasma Chemistry, Pugnochiuso, Italy, September 1989—“Direct a-Si:H and a-SiC:HPhotoCVD Deposition with a D2 Lamp and Dielectric Barrier Discharge Lamp” discloses using a coating of Fomblin oil to avoid film deposition on an MgF2 window.
P. Pastorino, G. Morello and S. Tamagno—Journal De Physique IV “Colloque C5, supplement au Journal de Physique 11, Volume 5, June 1995, “Amorphous Hydrogenated Silicon Nitride Deposited Mercury Photosensitization Chemical Vapour Deposition for Optoelectronic Applications” discloses using Fomblin oil on a quartz window of the chamber.
In both Kessler and Pastorino the Fomblin oil was used in a horizontal configuration, and when the oil is dissociated by the VUV photons it has to be replaced, or the window darkens. No oil bath is used.